fix(embed): make embed-mode framing opt-in

Embed-mode documents now keep the server's framing protection unless
GEV_EMBED_FRAME_ANCESTORS names the pages that may frame them (or * for any
page). The MCP Apps panel loads the app into its own page and needs no
framing. View answers are posted to the origin that sent the view rather
than to any origin.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

src/app/embed.js

@@ -249,9 +249,11 @@ export function installViews({
   // A framing page talks to the app across frames; an inline panel shares
   // the page with it and talks through the page's own window.
   const peer = isEmbeddedInline() ? windowRef : windowRef.parent;
-  const post = (message) => {
+  // Answers go back only to the origin that asked. An inline panel posts to
+  // its own window, and a sandboxed framing page has no origin to name.
+  const post = (message, origin = '*') => {
     if (peer && (peer !== windowRef || isEmbeddedInline()))
-      peer.postMessage(message, '*');
+      peer.postMessage(message, origin);
   };
   // Views apply one at a time, in the order they arrive.
   let queue = ready;
@@ -259,27 +261,37 @@ export function installViews({
     if (event.source !== peer || event.data?.type !== EMBED_VIEW_MESSAGE)
       return;
     const { id = null } = event.data;
+    const replyOrigin =
+      isEmbeddedInline() || !event.origin || event.origin === 'null'
+        ? '*'
+        : event.origin;
     let view;
     try {
       view = createView(event.data.view);
     } catch (error) {
-      post({
-        type: EMBED_APPLIED_MESSAGE,
-        id,
-        ok: false,
-        error: error.message,
-      });
+      post(
+        {
+          type: EMBED_APPLIED_MESSAGE,
+          id,
+          ok: false,
+          error: error.message,
+        },
+        replyOrigin,
+      );
       return;
     }
     queue = queue.then(async () => {
       if (signal?.aborted) return;
       const steps = await applyView(view, { viewer, dataManager, run, signal });
-      post({
-        type: EMBED_APPLIED_MESSAGE,
-        id,
-        ok: steps.every((step) => step.ok),
-        steps,
-      });
+      post(
+        {
+          type: EMBED_APPLIED_MESSAGE,
+          id,
+          ok: steps.every((step) => step.ok),
+          steps,
+        },
+        replyOrigin,
+      );
     });
   };
   windowRef.addEventListener('message', onMessage);